The use of carding machines for forming nonwoven webs of staple length fibers, oriented in the machine direction of web formation, are well known in the prior art. It is also known in the prior art to employ a fiber orienting and web spreading apparatus after the carding machines for the purpose of increasing the length of the formed web in the cross-machine direction of formation. The use of a fiber orienting and web spreading apparatus in conjunction with a carding machine is described in U.S. Pat. No. 3,772,107--Gentile, et al and in U.S. Pat. No. 3,940,216--Hinckley, both patents being assigned to the assignee of the present invention.
As disclosed in the above-mentioned patents to Gentile, et al and Hinckley, the web spreading and fiber reorienting section consists of a plurality of curved spreader rolls spaced from each other in the machine direction of web formation with their axles extending generally in the cross-machine direction of web formation. Typically, the web spreading section includes a set of about 12 spreader rolls. In one prior art configuration, the centerline of each spreader roll is a segment of a circle, each circle having the same radius of curvature. In another prior art configuration, the set of spreader rolls is broken down into groups of either 3 or 4 spreader rolls. The centerlines of the spreader rolls within a group are segments of a circle having the same radius of curvature, but the radius of curvature for each group of spreader rolls further downstream in the machine direction is greater than the radius of curvature of the preceding upstream group of spreader rolls.
With the prior art curved spreader rolls, the cross-machine width of the web coming out of a cardline is typically increased by about 45% at the commercial operating speed of web formation. Attempts to increase either the web formation speed or the amount of cross-machine spreading results in basis weight variations across the formed web that are not commercially acceptable. In particular, the basis weight of the central portion of the spread web is less than the basis weight of the edge portions of the spread web. Since the formed web is usually manufactured to meet a specified minimum basis weight, the extra basis weight at the end portions of the web represents an inefficiency in the manufacturing process.
It is believed that the basis weight variation caused by the prior art spreading sections is due to a combination of over-spreading in the central portion of the web and a "necking down," or lateral displacement of fibers toward the center of the web, as the formed web leaves the spreading section. Since the fibers at the edge of the formed web undergo a larger lateral displacement than the fibers at the central portion of the web, the necking down phenomenon also increases the basis weight of the edge portion of the formed web in comparison to the central portion of the web.
It is also known in the prior art to employ within a set of spreader rolls a single roll having a centrally located straight section to provide limited arresting of the spreading of fibers in the web.
It is, therefore, an object of this invention to provide an improved web spreading apparatus for use in the manufacture of nonwoven webs.
It is another object of this invention to provide a web spreading apparatus that results in a more even basis weight distribution across the width of a formed web.
It is yet another object of this invention to provide a web spreading apparatus that results in a more even basis weight distribution across the width of a formed web while increasing the length the web is spread in the cross-machine direction when compared to prior art spreaders.